Voltage breakdown control in strain gages



June 6, 1967 A. VAN LEEUWEN 3,324,435

VOLTAGE BREAKDOWN CONTROL IN STRAIN GAGES Original Filed June 1, 1965ANDREW VAAJ .EEEUWEAJ INVENTOR.

ATTORNEV United States Patent $324,435 VOLTAGE BREAKDOWN CONTROL INSTRAIN GAGES Andrew Van Leeuwen, Los Angeles, Calif., assignor toStatham Instruments, Inc., Los Angeles, Calif., .a corporation ofCalifornia Continuation of abandoned application Ser. No. 460,158,June 1. 1965. This application Oct. 20, 1966, Ser. No. 588,223

9 Claims. (Cl. 338-4) This application is a continuation of applicationSer. No. 460,158, filed June 1, 1965, now abandoned.

Strain gage transducers such as are shown in the following Stathampatents, employed as pressure transducers, have the reference volumefilled with low pressure helium, to reduce the deleterious thermaleffects resulting from the heating of the wires by the current appliedto the Wires. These patents are as follows: 2,778,624, 2,622,176,2,453,549 and 2,840,675.

The above transducers-are known as unbonded strain gage transducers.Other transducers also applicable as pressure transducers employ bondedtype strain gages attached to flexures which are deflected by reason ofthe applied pressure. In all of these transducers the pressure isapplied to a force summing means such as a diaphragm.

As is well known, the pressure applied to the pressure gage istranslated into a strain in the wires, to change their resistance. Thewires are connected into a Wheatstone bridge configuration. The currentwhich passes through the bridge from the excitation source causes aheating of the wires, resulting in a shift in the balance of the bridgeat zero pressure, and has other deleterious effects.

The introduction of a small amount of helium, the magnitude depending onthe pressure in the reference volume, effectively minimizes thedeleterious eifects of this heating. It has, however, as I have found,the dis-.-

advantage that the pressures of helium in the transducers of the abovetype, required to obtain the above eifect;

that there are no shorts between the current input ter-f minals whichexcite the bridge and the ground, i.e., the case of the transducer. Thisis done usually by applying a high voltage between the case and theinput terminals of the transducer. It is desired that ideally thisconstitutes an open circuit, but in practical effect it has been foundsufficient that the leakage current shall be of the order of aboutone-half milliampere, at voltages in the neighborhood of 300 to 400volts, at 60 cycles.

It has been found that, employing helium in the reference volume, in theranges of about .1 to about p.s.i.a., in transducers used as pressuregages for pressures ranging from about 1 pound up to 5,000 pounds, thevoltage at which the above leakage current occurs is so high that afalse indication of a short occurs at voltages as stated above.

I have now found that, if I add to the helium a small amount of afluorocarbon gas at partial pressure which is substantially less thanthe partial pressure of the helium in the reference volume, theinsulation value of the gas in the reference volume becomes sufficientlyhigh so that, at suitably high test voltages, the leakage current willbe not more than a suitable level. In such systems the transducers maybe tested at suitable voltage levels 3,324,435 Patented June 6, 1967 7and, if not shorted for other reasons, will not show an undesiredcurrent flow through the gas.

It is, therefore, an object of my invention to produce electricalresistance strain wire pressure transducers in which the referencevolume is filled with low pressure helium containinga fluorocarbon at apartial pressure substantially less than the partial pressure of thehelium, the total pressure being in the range of from about 0.01 toabout 20 p.s.i.a., and the partial pressure of the helium may be in therange of from about 1000 to about 10 times the partial pressure of thefluorocarbon. In low pressure gages, for example, gages in the range of0 to200 p.s.i.a., where the total pressure in the reference volume iscomposed substantially entirely of helium at pressures in the range ofabout .1 to about 1 p.s.i.a., the partial pressure of the fluorocarbonmay range from about 0.001 to about .1 p.s.i.a, a suitable partialpressure of the fluorocarbon being within the range of about .01 toabout .05 p.s.i.a.

These and other objects of my invention will be more fully described byreference to the drawing, of which:

FIG. 1 shows one type of pressure transducer, that shown in StathamPatent 3,058,348, which is here shown merely for illustrative purposes.It must be remembered that any pressure transducer in which a sealedreference volume may be employed may be used.

In FIG. 1, 1 is the case, which is hermetically and rigidly connected tothe pressure input adapter 2. A diaphragm 3 is connected to the inletadapter 2, and sealed at the edges of the diphragm. A post 9 isconnected to the center of the diaphragm. The case carries a frame 5, inthe form of a plate, which is supported upon the pressure input adapter2. The plate carries two insulated posts 8 and two springs mounted oninsulated posts 6'. The wires 7 are connected to posts 8 and to thesprings6 and to a post 9'.

The metallic springs 6 are connected to terminals 10', and theinsulate-d posts 8 and 9' are also connected to suitable terminals 10',as is fully disclosed in the said Statham patent. The reverse side ofthe frame 5 carries a .similar wire arrangement; The wires are connectedinto a form of Wheatstone bridge, as shown in FIG. 2, by suitableconnections to the terminals 10 and to the ter- I minals 10, as is fullydisclosed in said Statham patent.

The case carries a terminal cover 16, carrying the ter- .minalconnections 10, and has a sealing plate 17, carry ing solid terminals18-to which the conductor wires are connected, and has a central opening19. The central opening 19 in position with the case 16 removed, thetransducer is introduced into a vacuum chamber. A suitable gas ormixture of gases is introduced through the opening 19, which is sealedwith a solder or a weld seal at 21, to hold the gas under suitablepressure in the reference volume 4. The cover 16 is then placed on thecase.

To test the trans-ducer to determine the voltage at which a current flowof /2 milliampere or any other suitable amperage will flow from theterminals 10 and the conductor 11 through the wires 7 to the case 1,through the gas in the reference volume 4, the terminals 10 areconnected together, as shown in FIG. 2, and the case is connectedthrough a resistance 13 to a potential source 12. Suitable voltage isapplied at 12 and the current measured by measuring the potential dropacross the resistance 13.

In such a transducer, in which the evacuated reference volume 4 isfilled with helium alone, the voltage at which the .5 milliampere flowsis given in the following table. In column 1 is given the pressure ofthe helium in the reference volume 4. Column 2 is the voltage at which acurrent fl-ow of .5 milliampere occurs through the resistance 13, at 60cycles applied at 12, with helium only at the indicated pressure. Column3 is the voltage for 3 such current flow, when 0.02 p.s.i.a. of CClF isadded to the helium in the reference volume 4, to give a total pressureas indicated in column 1.

TABLE I Helium only Helium and CClFa P.s.i.a.

curo UUQNJOS It will be seen that a significant increase in theinsulating value is obtained. This is particularly important in theregions of .1 to .5 p.s.i.a., where the reference volume is under lowpressure as in absolute pressure gages. In all such pressure gages it isdesired that the pressure in the reference volume be as small a fractionas possible of the minimum pressure which the gage is to report.

It will be seen that a significant increase in the resulting voltage atwhich .5. milliampere flow is obtained results when the fluorocarbon gasis added to the helium. This is particularly important in the regions ofabout .1 to .5 p.s.i.a., as used in low pressure and medium pressureabsolute pressure gages in the range of 1 to 200 p.s.i.a. necessary inorder that the pressure in the reference volume dose not introduce anunreasonable stiffness into the system and does not raise the pressurerequired to move the bridge unbalance from zero.

In all such pressure gages it is desired that the pressure in thereference volume be as small a fraction of the total pressure which thegage is to report, whether it be high or low pressure, that is, whetherfrom 1 to 5,000

pounds.

The fluorocarbons which I may use include:

Trifluorochloromethane CClF Octofluorocyclobutane C F Hexafluoroethane C1 and mixtures thereof.

I prefer to employ CClF or C F or mixtures thereof. Of these, C 1 is thepreferred fluorocarbon, since it is the most stable under the aboveconditions.

I may thus employ the above gases together with helium 4 at a totalpressure of from about .1 to about 5 p.s.i.a., in which the partialpressure of the fluorocarbon gas is from about 0.01 to about .5p.s.i.a., and in which the total pressure is not in excess of about 1%of the lowest pressure which the gage is intended to report.

While I have described a particular embodiment of my invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention, as set forth in the appended claims.

I claim:

1. A pressure transducer comprising a case, a force transmittingflexure, a sealed reference volume, an electrical resistance strain wiremounted in said reference volume and operatively connected to saidflexure, helium in said reference volume, a fluorocarbon gas in saidref-.

erence volume at a partial pressure substantiall less than the pressureof said helium, the total pressure in said reference volume beingsubstantially equal to the pressure of said helium and saidfluorocarbon.

2. The transducer of claim 1, in which the fluorocarbon is CClF 3. Thetransducer of claim 1, in which the fluorocarbon is C2126.

4. The transducer of claim 1, in which the partial pressure of thehelium is in the range of from about 1000 to about 10 times the partialpressure of the fluorocarbon, and the total pressure in the referencevolume is substantially equal to the pressure of said helium and saidfluorocarbon.

5. The transducer of claim 4, in which the fluorocarbon iS CC1F3.

6. The transducer of claim 4,

7. The transducer of claim 1, in which the partial pressure of thehelium is from about .1 to about .5 and the partial pressure of thefluorocarbon is from .01 to .05 p.s.i.a., the total pressure in saidreference volume being substantially equal to the pressure of saidfluorocarbon and said helium.

8. The transducer of claim 7, in which the fluorocarbon is CC1F3.

9. The transducer of claim 7, in which the fluorocarbon is C2126.

References Cited UNITED STATES PATENTS 3,058,348 10/1962 'Statham 33843,242,448 3/1966 Paine et al 3384 RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner.

in which the fluorocarbon

1. A PRESSURE TRANSDUCER COMPRISING A CASE, A FORCE TRANSMITTING FLEXURE, A SEALED REFERENCE VOLUME, AN ELECTRICAL RESISTANCE STRAIN WIRE MOUNTED IN SAID REFERENCE VOLUME AND OPERATIVELY CONNECTED TO SAID FLEXURE, HELIUM IN SAID REFERENCE VOLUME, A FLUOROCARBOM GAS IN SAID REFERENCE VOLUME AT A PARTIAL PRESSURE SUBSTANIALLY LESS THAN THE PRESSURE OF SAID HELIUM, THE TOTAL PRESSURE IN SAID REFERENCE VOLUME BEING SUBSTANTIALLY EQUAL TO THE PRESSURE OF SAID HELIUM AND SAID FLUOROCARBON. 